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Regenerative Braking

Page history last edited by Jason 13 years ago

Regenerative Braking

What is Regenerative Braking?

A regenerative brake is an energy recovery mechanism which slows a vehicle by converting its kinetic energy into another form,

which can be either used immediately or stored until needed. This contrasts with conventional braking systems, where the excess kinetic energy is converted to heat by friction and therefore wasted. The most common form of regenerative braking takes places i electric or hybrid/electric cars. In these types of cars the energy typically lost to heat from the friction of the brake pads can be transfered into electricity to recharge the battery of the car.

The Quick History Behind Regenerative Braking

Regenerative breaking was first used over 100 years ago! Louis Antoine Krieger came up with a front-wheel drive conversions on a horse-drawn cab in 1903. It has drive motors in each of the front wheels that were used for regenerative breaking. Regenerative braking was still in its development stages and didn't truly start appearing until the 1930's. During this time trains and trolley cars began installing regenerative braking systems. Regenerative braking has really taken off in like last 5 to 10 years with the development of electric and hybrid/electric cars but it is also being used in Formula 1 racing. Some systems have become so effective that they actually create more energy than they use. This is true in the case of the train that travels the steep and dangerous Surami Pass. In Scandinavia the Kiruna to Narvik railway carries iron ore from the mines in Kiruna in the north of Sweden down to the port of Narvik in Norway. The rail cars are full of thousands of tons of iron ore on the way down to Narvik, and these trains generate large amounts of electricity by their regenerative braking. From Riksgränsen on the national border to the Port of Narvik, the trains use only a fifth of the power they regenerate. The regenerated energy is sufficient to power the empty trains back up to the national border. Any excess energy from the railway is pumped into the power grid to supply homes and businesses in the region, and the railway is a net generator of electricity.

Normal Braking vs. Regenerative Braking

Normal braking in your typical modern car is done by disk brakes. Disc brakes work by having a rotor attached to the wheel and through hydraulic pressure the caliper pushes the brake pads against the rotor not the actual wheel. But as the rotor slows down via friction with the brake pads the wheels slow down. A LOT of energy is wasted from this process; the friction causes the kinetic energy of the tires spinning to dissipate into the air as heat. However these brakes have been proven effective and other than the fact that it wastes energy have next to no limitations.

Regenerative braking in your electric or hybrid/electric car is done by 20% mechanical breaking (friction braking) and 80% regenerative braking. The basic science behind regenerative braking is that to slow the wheels down the electric motor that powers the car is simply turned in reverse; this slows the wheels down and the electric motor acts as a generator creating electricity from the kinetic energy that is sent back to the car's battery. Regenerative breaking doesn't waste nearly as much energy as normal braking but it has some limitations:

The most obvious limitation is that in order for regenerative braking to work the car must have an electric motor that is used to power the car which means it must be electric or hybrid/electric and these cars vastly out price standard internal combustion cars.
Regenerative breaking does not work well at very lower speeds because the regenerative braking can only slow the car down it can't actually bring it to a complete halt because it cant lock the rotor in place so friction brakes must still be used.
Regenerative braking doesn't work well at high speeds; it isn't nearly as effective at slowing the car down as standard disk brakes so at high speeds or in emergency situations friction brakes are still required to bring the car to a complete stop in the shortest amount of time.
Most cars are two wheel drive and because the electric motor only powers the drive wheels there is only braking on the two drive wheels. In situation where the road is slippery for example and more control is needed during breaking friction brakes are required so there is braking on all four wheels.

It is obvious that regenerative braking will let less energy go to waste than standard friction brakes but because friction brakes are still required with regenerative braking a lot of energy is still wasted.

A more In Depth look at Regenerative Braking

The basis of regenerative braking has to primarily do with motors and generators (Remember this from grade 11 physics?). Motor/generators are really one device that can run

in two opposite modes. Contrary to what folks sometimes think, that does not mean that the two modes of the motor/generator run backwards from each other (that as a motor the device turns in one direction and as a generator, it turns the opposite direction). The shaft always spins the same way. The "change of direction" is in the flow of electricity. As a motor it consumes electricity (flows in) to make mechanical power, and as a generator, it consumes mechanical power to produce electricity (flows out). If you can remember a simple motor has a coil of wire that can rotate in a magnetic field. The current in the coil is supplied via two brushes that make moving contact with a split ring. The coil lies in a steady magnetic field. The forces exerted on the current-carrying wires create a torque on the coil.

Now to the actual regenerative braking. An electric motor in a car is much more complicated than the basic motor shown but follows the same basic idea this means that when the car is accelerating the electric motor consumes energy from the batteries to move the car but when the electric motor is turned in reverse it uses the kinetic energy from the spinning tires and uses it to generate electricity and recharge the same batteries that power the motor. Regenerative braking is the process of turning the electric motor in reverse. It slows the wheels down,the braking part, and generates electricity using the same motor that accelerates the car, but in reverse.

Effectiveness of Regenerative Braking

Regenerative braking saves between 30%-50% of the energy used while braking. This may seem like a relatively low percentage but considering how much you use your brakes that amount to a lot of energy saved when you compare it to friction brakes. However the main purpose of an electric or hybrid electric car is to be able to travel without having to use gasoline and avoid releasing carbon emissions. In purely electric cars a major problem is battery life; in order for an electric motor to power a car it requires a lot of electricity and because of this purely electric cars have trouble with long trips. They could increase the number of batteries but at the cost of adding weight so regenerative braking is a huge help to this problem; it allows the battery to constantly recharge bits at a time improving the battery life significantly. Similarly in hybrid/electric it allows the car to run longer on electricity and at higher speeds without having to use the internal combustion engine. This can result in you saving up to 800$ in gas every year!

Have you been Paying Attention?

A quick question to see if you have learned anything from reading this wiki!

A basic motor and a basic generator are the same thing but opposites, whats the difference?

Highlight below for answer!

A motor converts electrical energy to mechanical energy where a generator converts mechanical energy to electrical energy. A motor has current flown through coil in presence of magnetic field resulting in motion where a generator where as has a coil is rotated in a magnetic field causes generation of electricity.